1. Field of the Invention:
The present invention relates to a fuel cell stack including a box-shaped casing and a stack body provided in the casing. The stack body is formed by stacking a plurality of unit cells. Each of the unit cells includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes.
2. Description of the Related Art:
For example, a solid polymer fuel cell employs a membrane electrode assembly which includes an anode and a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a fuel cell (unit cell) for generating electricity.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. A gas chiefly containing oxygen or air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
Generally, a predetermined number of, e.g., several tens to several hundreds of fuel cells are stacked together to form a fuel cell stack for achieving the desired level of electricity in power generation. Components of the fuel cell stack need to be tightened together reliably under pressure so that the internal resistance of the fuel cell does not increase, and the sealing performance for preventing leakage of reactant gases is maintained.
In this regard, for example, a fuel cell stack disclosed in Japanese Laid-Open Patent Publication No. 2001-135344 is known. As shown in FIG. 34, the fuel cell stack includes a stack body 2 formed by stacking a plurality of unit cells 1. At opposite ends of the stack body 2 in the stacking direction, end plates 3 are provided. Further, auxiliary plates 4a, 4b are provided outside the end plates 3. A pair of tightening bands 5 is provided along both sides of the stack body 2. At ends of the tightening bands 5 and the auxiliary plates 4a, 4b, cylindrical coupling members 6 are provided. The coupling members 6 have holes arranged in a straight line. A metal pin 7 is inserted into each of the coupling members 6 for coupling the tightening bands 5 and the auxiliary plates 4a, 4b together.
According to the disclosure, a plurality of bolts 8 are screwed into the auxiliary plate 4a. A plurality of belleville springs 9 are provided on the auxiliary plate 4b. Therefore, when the bolts 8 are screwed into the auxiliary plate 4a, the end plates 3 are pressed downwardly, and the belleville springs 9 on the auxiliary plate 4b are compressed. Thus, the required tightening pressure is applied to the stack body 2 through the pair of end plates 3.
In Japanese Laid-Open Patent Publication No. 2001-135344, the metal pins 7 are fitted into both of the coupling members 6 of the tightening bands 5 and the coupling members 6 of the auxiliary plates 4a, 4b. Therefore, the distance between the auxiliary plates 4a, 4b does not change. Thus, when the tightening load applied to the fuel cell stack is reduced by aged deterioration or the like, it is necessary to tighten the plurality of bolts 8 for maintaining the desired tightening load.
However, in Japanese Laid-Open Patent Publication No. 2001-135344, in order to adjust the tightening load applied to the fuel cell stack, the bolts 8 as dedicated components for tightening are required. Therefore the additional dedicated components increase the weight of the fuel cell stack, and the size of the fuel cell stack in the stacking direction of the unit cells 1 disadvantageously.
As a solution of the above problem, for example, a fuel cell stack disclosed in Japanese Laid-Open Patent Publication No. 2002-298901 is known. In the fuel cell stack, current collecting electrodes (terminal plates) are provided on the outside of a stack body formed by stacking a predetermined number of unit cells. Further, end plates are provided outside the terminal plates. The end plates are coupled to a casing by a hinge mechanism. The casing includes a plurality of panels (side plates) provided on upper, lower, left, and right sides between the end plates.
Therefore, in Japanese Laid-Open Patent Publication No. 2002-298901, the bolts 8 disclosed in Japanese Laid-Open Patent Publication No. 2001-135344 are not required. Further, it is possible to use thin end plates. Thus, it is possible to achieve reduction in the overall size and weight of the fuel cell stack easily.
In Japanese Laid-Open Patent Publication No. 2002-298901, for example, a relatively large gap may be formed between the side plates of the casing and the sides of the stack body. Therefore, when an impact is applied to the fuel cell stack, positional displacement may occur in the stack body in the stacking direction, or the stack body may be vibrated undesirably. If the gap between the side plates and the sides of the stack body is excessively small, the side plates and the stack body contact unnecessarily.
Further, the panels of the casing have a flat shape. For example, if a load is applied to the fuel cell stack in a twisting direction, or if vibrations are generated, deformation may occur in the panels. Thus, the position of the stack body may be displaced in the stacking direction. In this case, for example, the sealing performance is degraded undesirably.
In view of the above, for example, a rib-panel having ribs may be adopted to increase the rigidity of the panel. However, since the tightening load is applied to the stack body in the stacking direction, when the load in the stacking direction is applied to the rib-panel, the rib-panel may be deformed undesirably by the load.